1. Field of the Invention
This invention relates to a motion adaptive noise reduction filter that removes noise on a moving image, and thereby achieves a high quality moving image. It further relates to a motion compensated interframe coding system using this filter.
2. Description of the Prior Art
In a conventional technique for efficiently removing noise from a moving image, an interframe low pass filter is constructed by making use of the fact that there is a high interframe correlation between image signals, but not much interframe correlation between noise. This technique efficiently removes noise from signal components in stationary images or slowly moving parts of images.
In another conventional method, known as the post-processing scheme, a filter is introduced to remove coding noise in motion compensated interframe coding where the signals are assumed to be compressed.
This second method, as shown in FIG. 8, may for example comprise a motion compensated interframe coding system wherein an input signal xi is encoded, and a prediction error signal (.epsilon.i+ni) with a quantization noise ni is obtained. This is reproduced as a prediction signal yi2, the noise is removed by a noise reduction filter consisting of an interframe closed loop, and a final output signal yi3 is obtained. Herein, Z.sup.-1 is a delay factor representing the frame delay.
According to the first method, however, either there was no motion compensation, or even if such compensation was performed, it was applied to screen units. As a result, motion compensation was inadequate, the moving image was blurred, and noise could not be completely eliminated. Further, if the aforesaid interframe low pass filter was applied to areas where the original signal varied largely between frames, i.e. to areas with rapid motion, much of the original signal was lost, and judder or other distortion occurred. For example, if it were applied to the image of a man standing and waving his hand vigorously up and down, noise could be efficiently removed from image parts where the man's body was still, but the waving hand was affected by judder or other distortion.
Further, according to the aforesaid second method, the motion compensated interframe coding system requires a loop to reproduce the prediction error signal (.epsilon.i+ni) as the prediction signal yi2, and it also requires a multiplier in the loop of the noise reduction filter that leads to an increase of hardware. Moreover, the second method does not contribute in any way to reducing the amount of information to be transmitted, i.e. to the amount of information in the prediction error signal .epsilon.i.